MABEE, P.M.; CROTWELL, P.L.*; BIRD, N.C.: Models to explain directions of development and evolution of fin skeletons.

Data on development of radials and rays of median (dorsal, anal, caudal) and paired (pectoral, pelvic) fins were compared within a phylogenetic framework. We found broadly conserved phylogenetic patterns in directions of development within the different median and paired fins. Variability within closely related taxa (e.g. Percoidei) demonstrates the evolution of regional specialization and new levels of independence within formerly tightly integrated fin modules. We consider a module to be an element that develops as an independent unit and undergoes coordinated changes in evolution. For example, the dorsal and anal fins develop in similar ways and undergo coordinated changes inevolution, thus may be classified as a module. Previously unrecognized patterns in median fin development contribute to our understanding of fin module development and evolution, and influence construction of genetic models to explain directional phenomena. Based on constraints observed within these taxa, models consistent with observed directional patterns were developed. Hox gene activation or repression may account for location of median fins and direction of radial formation within. Or, perhaps in analogy to butterfly wing eyespot development, specific genes are expressed in a pattern of symmetrical nesting, thus patterning fin location and development. Modern genetic techniques will permit researchers to test the models presented here, consequently significantly increasing our understanding of the evolution and development of fin modules.